1. Field of the Invention
The present invention relates to delay-locked loop circuits. In particular, the invention relates to reducing the body effect of circuit elements in delay-locked loop circuits.
2. Description of the Related Art
In the design of delay-locked or phase-locked loops, the phase detector is an important component. The basic idea of the loop is to measure the phase difference between two clock signals, or more specifically, to measure the timing difference between two rising edges of two clock signals, and then to feed back this timing difference information to a component such as a voltage controlled delay chain. The delay chain adjusts the timing delay of one (or both) of the clock signals, thereby bringing the timing difference to zero. A timing difference of zero is also called a phase alignment of zero, which means that the clock signals transition at the same time. In such a case the output of the phase detector should indicate that no phase adjustment is necessary.
However, there are two situations in which poor phase detector design can lead to phase alignment errors. First, a poor design may cause the phase detector to feed back adjustment information even when the two clock edges are already in alignment. Second, a poor design may cause the phase detector to misdetect zero phase alignment and not feed back adjustment information, even when the two clock signals are not in alignment. These errors are called static phase alignment errors.
One way in which the design of the phase detector contributes to static phase alignment error is when the clock signals must drive different numbers of transistors, some of which have body effect and some of which do not. For example, FIG. 1 shows an XOR phase detector circuit. Node 17 supplies power and node 19 is connected to ground. Node 18 connects the substrates of the transistors to ground. Clock signal 11 drives only one gate of transistor 13 but clock signal 12 drives two gates of transistors 14 and 15. So even if the designer sizes the transistors 13-15 to get equal loading for clock signals 11-12, clock signal 11 drives transistor 13 without body effect but clock signal 12 drives transistors 14-15 which do have body effect.
This body effect results from a voltage difference between the substrate and the source of each transistor. Transistor 13 has no body effect because its substrate is connected to node 18 and its source is connected to node 19, both of which are ground nodes. On the other hand, transistor 14 has body effect because, although its substrate is connected to node 18, its source is connected to the drain of transistor 13. Similarly, transistor 15 has body effect because its source is connected to the drain of transistor 16. Thus, it is easier for clock signal 11 to turn on its gate than for clock signal 12 to turn on its gates. The phase detector will then sense clock signal 11 differently than clock signal 12. This difference will contribute to the system static phase alignment error.
Another contribution to the error is the design of the charge pump. The output of the charge pump drives an adjustment circuit such as a voltage controlled oscillator (VCO). For generation of a good, low jitter VCO output, a small charge pump output ripple is needed. However, for many existing charge pumps, the output node is connected to rapidly switching PMOS and NMOS transistors, which will generate noise at the output node and lead to a large ripple.
A third contribution to the error is the design of the transconductance stage. FIG. 2 illustrates a typical transconductance circuit. The difference between currents I.sub.1 and I.sub.2 is proportional to the difference between the gate-to-source voltages of M1 and M2. However, this assumes that the gate-source voltages of M5 and M7 are both V.sub.B. This is not correct if transistor body effect is considered, even when M5 and M7 are the same size and source the same current. This is because the threshold voltage of M5 is larger than that of M7 because the source-substrate voltage of M5 is not zero. This means the gate-source voltages of M5 and M7 cannot both be equal to V.sub.B. The same is true for M6 and M8.
Body effect occurs when the potential of the substrate of a MOSFET is different from the source potential. The body effect increases the threshold voltage of the MOSFET. The body effect contributes to nonlinearity. A way is needed to overcome the body effect in both the phase detector and the transconductance circuit in a delay-locked loop circuit.